The levels of norepinephrine, epinephrine, and dopamine and their metabolites in various body fluids reflect the activity of the neurones from which these neurotransmitters are released. Urinary catecholamine metabolites provide an index of overall synthesis in the body while the relative proportion of normetanephrine appears to be related to the functional activity. Although plasma levels of norepinephrine reflect the responses of the peripheral sympathetic nervous system it is necessary to consider removal rates of the catecholamine. Cerebrospinal fluid levels of catecholamine metabolites can be used to assess central nervous system metabolism if appropriate corrections are made for the contribution from plasma. A more thorough understanding of neurotransmitter metabolism leads to more rational approaches to therapy.